(a) Field of the Invention
This invention relates to an image forming lens system for microscopic objectives of an infinite distance design to be used for the observation of a metal phase system or the like.
The conventional image forming lens systems of this kind is mostly of such formation as is shown in FIG. 1. That is to say, as in FIG. 1, it consists of two lens elements L.sub.1 and L.sub.2 between which there is only a slight air space. Therefore, in the conventional image forming lens system, the exit pupil is in a position so comparatively far away from the lens element L.sub.1 is to the left in the drawing that the light bundle will become so large as to produce a phenomenon of a part of the light bundle being cut after this image forming lens system. In such case, in order that the light bundle may not be cut, the optical system must be made larger, for example, the lens diameter must be made larger. Further, the pupil will separate so far away from the lens that asymmetrical comas will be generated. Further, as this kind of image forming lens system is a lens system for forming the image of a microscopic objective designed for an infinite distance, it has not only an image forming action but also an action of determining the position of the exit pupil of the objective within a predetermined range. Further, if the image forming position is far away, the observing position will be away from the microscope body (the eye point will become higher) and will become so hard to see that it will be necessary to form the image as near as possible. In fact, from the requirement in the structure of the microscope body, the image forming position is not the image position (back focus position) determined most naturally on the basis of the focal distance of the lens system but must be shorter than that and is of a lens system of a so-called telephoto ratio P=0.7 to 0.8.